Enhanced oil recovery encompasses many methods for increasing the recovery of oil remaining in a reservoir after the natural pressures are insufficient for economic production. Primary recovery from a reservoir often produces only 10 to 30 percent of the oil present in the reservoir. Various supplemental methods have been developed to recover at least a portion of the remaining oil. Several water and chemical flooding methods are common techniques in enhanced oil recovery. These can involve the injection of a fluid such as water from one or more injection wells spaced at some distance from one or more producing wells to force additional oil toward the producing wells. The injection fluid can further include one or more chemicals, injected either together or in sequence to the reservoir. Some recovery methods involve the injection of surfactants, while a method such as caustic flooding uses inexpensive chemicals such as sodium hydroxide which can react in situ to provide surfactants and increase oil production through mechanisms such as lowering the interfacial tension between the injected fluid and the oil.
Caustic flooding or more generally alkaline flooding is a process in which the pH value of the injected flood water is increased to a value above about 8, more usually above about 10, and most usually between about 10 and 14 by adding sodium hydroxide or another alkaline chemical thereto. The most common alkaline chemical used is sodium hydroxide because of its ready availability and low cost. Other alkaline chemicals, which can accomplish the same result, include sodium carbonate, sodium bicarbonate, sodium silicates, sodium borates, sodium phosphates, potassium hydroxide, potassium carbonate, potassium bicarbonate, potassium silicate, potassium borates ammonia, and selected amines such as methylamine or polyethylenimine. Benefits of alkaline flooding can include improvement of surfactant floods and wettability alteration.
Alkaline flooding processes are complicated by the fact that as the alkaline flood water is injected into the reservoir, multivalent metal cations, particularly divalent metal cations present in the injection water or reservoir water can precipitate. The troublesome divalent metal cations are typically calcium, magnesium and barium. The precipitation of divalent metal salts, e.g., divalent metal hydroxides or divalent metal carbonates can plug the injection wall or the reservoir in the vicinity of the injection well. Such plugging can hinder the injection of further alkaline flood water. While the problem of calcium hydroxide and magnesium hydroxide scales has been generally overlooked, U.S. Pat. No. 4,466,892 does describe adding lignosulfonate to alkaline injection water containing divalent metal cations to prevent substantial precipitation of hydroxides, such as calcium hydroxide.
The prior art has suggested softening the injection water through ion exchange to remove the divalent metal cations whereby to prevent precipitation within the injection well. Another suggestion has been to remove precipitated hydroxides or carbonates from the injection water by filtration before the alkaline flood water is injected into the reservoir. While this can eliminate plugging of the injection well, it may not prevent plugging within the reservoir in the vicinity of the injection well as aqueous water within the reservoir, e.g., connate water, can also contain significant levels of dissolved divalent metal cations. Therefore, still another suggestion has been to inject a preflush of water substantially free of divalent metal cations into the reservoir in order to condition the reservoir and reduce or prevent plugging in the vicinity of the injection well.
While it is recognized that plugging of a reservoir in the immediate vicinity of the injection well is detrimental, it is further recognized that reservoirs are typically heterogeneous, i.e., the reservoirs are comprised of stratified layers of varying permeability and can contain fractures, cracks, fissures, or streaks of varying permeability that cause the injected flood water to advance through such reservoirs nonuniformly. The injected aqueous solution will tend to flow or channel into the areas of higher permeability and thus bypass those portions of a reservoir having lower permeability. This can result in the reduced recovery of oil. Various methods have been suggested for adjusting the permeability of a reservoir including the precipitation of an insoluble material within the reservoir or the reaction of at least two materials whereby to plug the highly permeable portions of the reservoir. Plugging the highly permeable portions of the reservoir forces the aqueous solution into the lower permeable areas and can result in increased oil recovery.
It is desirable to have an improved alkaline injection fluid for enhanced oil recovery which would prevent substantial precipitation and plugging by divalent metal salts, e.g., divalent metal hydroxides and divalent metal carbonates in the injection well or in the immediate vicinity of the injection well within the reservoir. Furthermore, it is desirable that the alkaline injection fluid allow the formation of precipitates such as divalent metal hydroxides away from the immediate vicinity of the injection well whereby to alter the permeability and flow-profile of the reservoir.